It's literally all about the moment-to-moment gameplay and the experience of fighting (and perhaps mastering) these marvellously animated, expressive, varied monsters. The hook is not in levelling up, because you don't gain any XP besides your own knowledge, but in being able to fight harder, weirder and wilder monsters.
It's also about the gear, definitely. It's not a loot game, but it is about seeing a weapon upgrade path and deciding that it looks and/or sounds pretty cool and so fighting a monster a couple more times to get the parts for it. It is a bit of a grind much later on but it's actually pretty easy at first to get a load of different weird gear and weapons.
There is a single player campaign but it's always tripe, and more just a tutorial for the MP stuff. It's actually very literal and honest in what it is, there's absolutely no fluff and no pretension. It's almost arcade in its purity. It's like, if you don't enjoy playing it for the sake of the gameplay, it doesn't try to dupe you or tempt you with any other bullshit. It's just fucking fun and satisfying learning to fight the monsters.
The combat is the meat of the game and, as I said up thread, there's a really high skill ceiling to it. Like, unbelievably so. I can labour my way through a half hour fight with my massive upgraded hammer and three other numbskulls, but a better, more knowledgeable player can take it down solo in half the time with starting gear. Every hit feels satisfying because it's actually quite hard to make clean contact. That's also why part of the hook because it just feels fucking good to play. Every time I sprint in, leap off my steed and smash my hammer square into some monster's skull, I instantly clip it because it gives me a burst of dopamine.
